fix(runtime): fetch string_from_header must reject handle-band ids (doctor/mcp SIGSEGV) (#5560)

The doctor / mcp-list startup segfault (EXC_BAD_ACCESS at the fetch-handle
address, e.g. 0x40005/0x40006) root-causes to perry_stdlib::fetch::
string_from_header dereferencing a native registry id as a *StringHeader.

lldb on a debug bundle (doctor):
  frame #0 perry_stdlib::fetch::string_from_header  ldr w9,[x1,#0x4]  x1=0x40002
  frame #1 js_fetch_with_options
  frame #2 global_this_fetch_thunk

fetch() was called with a non-string first argument — a Request/Headers object
(a Web Fetch handle, registry id in [0x40000, 0xE0000)). The codegen passes the
bare handle id into the url_ptr *StringHeader slot of
js_fetch_with_options(url_ptr, method_ptr, body_ptr, headers_json_ptr), and
string_from_header reads (*ptr).byte_len at id+4 -> deref of unmapped low
address -> SIGSEGV. Its only guard was `ptr < 0x1000` (the TAG_UNDEFINED 0x1
remnant); a 0x40002 handle clears that floor.

Non-deterministic for mcp list because whether the id+4 page is mapped at the
call depends on heap/page layout: when mapped, byte_len reads garbage and fetch
proceeds to the intended error (clean rc=1); when unmapped, SIGSEGV.

Fix: reject the whole handle band (is_handle_band, < 0x100000) in
string_from_header so a native handle is treated as 'not a string' (None)
instead of dereferenced — same robustness pattern as the inline .length /
IC-miss / class-field-guard band gates. Adds a regression test.

This is the doctor/mcp wall and is INDEPENDENT of the alias-new fix
(a9e41e163) and of the inline-.length handle-band fix (175fda498) — both touch
different code paths; a9e41e163 merely advanced doctor far enough to reach this
fetch() call.

Co-authored-by: Ralph Küpper <ralph2@skelpo.com>

Author: proggeramlug

crates/perry-stdlib/src/fetch/mod.rs

@@ -120,6 +120,32 @@ mod tests {
         assert_ne!(native_id as usize, id);
         crate::common::drop_handle(native_id);
     }
+
+    /// `string_from_header` must treat a handle-band value (a Fetch / native
+    /// registry id, not a `StringHeader` pointer) as "not a string" and return
+    /// `None` WITHOUT dereferencing it. Regression for the doctor / mcp-list
+    /// startup SIGSEGV: `fetch()` called with a non-string first argument (a
+    /// `Request`/`Headers` object) passed the bare handle id into the
+    /// `url_ptr` `*StringHeader` slot, and reading `(*ptr).byte_len` at `id+4`
+    /// dereferenced an unmapped low address.
+    #[test]
+    fn string_from_header_rejects_handle_band_ids() {
+        use perry_runtime::value::addr_class;
+        for &id in &[
+            1usize,                                  // common native handle
+            addr_class::FETCH_HANDLE_BAND_START,     // 0x40000
+            addr_class::FETCH_HANDLE_BAND_START + 2, // a fetch handle id
+            addr_class::HANDLE_BAND_MAX - 1,         // 0xFFFFF
+        ] {
+            assert!(addr_class::is_handle_band(id));
+            // Must return None without dereferencing the bogus pointer.
+            let r = unsafe { string_from_header(id as *const StringHeader) };
+            assert!(
+                r.is_none(),
+                "handle-band id {id:#x} must be rejected, got {r:?}"
+            );
+        }
+    }
 }
 
 struct StreamState {
@@ -198,6 +224,20 @@ pub(crate) unsafe fn string_from_header(ptr: *const StringHeader) -> Option<Stri
     if ptr.is_null() || (ptr as usize) < 0x1000 {
         return None;
     }
+    // A handle-band value (`< 0x100000`: Web Fetch Headers/Request/Response/Blob
+    // ids, net/http small handles, zlib/proxy ids) is a registry id, NOT a
+    // `StringHeader` pointer. It reaches here when `fetch()` is called with a
+    // non-string first argument such as a `Request`/`Headers` object — the
+    // codegen passes the bare handle id into the `url_ptr` `*StringHeader`
+    // slot. Reading `(*ptr).byte_len` at `id + 4` then dereferences an
+    // unmapped low address → SIGSEGV (the doctor / mcp-list startup crash at
+    // the fetch-handle address). The `< 0x1000` floor above only catches the
+    // TAG_UNDEFINED `0x1` remnant; widen it to the whole handle band so any
+    // native handle is treated as "not a string" (`None`) rather than
+    // dereferenced.
+    if perry_runtime::value::addr_class::is_handle_band(ptr as usize) {
+        return None;
+    }
     let len = (*ptr).byte_len as usize;
     let data_ptr = (ptr as *const u8).add(std::mem::size_of::<StringHeader>());
     let bytes = std::slice::from_raw_parts(data_ptr, len);